Table 3.3: TS and VS-contents of green plants
(Source: Memento de l'agronome 1984)
Material
Rice straw
Wheat straw
Corn straw
Fresh grass
Water hyacinth
Bagasse
Vegetable residue
TS
(%)
89
82
80
24
7
65
12
VS
(% of TS)
93
94
91
89
75
78
86
Vegetable waste
Crop residue and related waste such as straw, cornstalks, sugar-beet leaves, etc. are often used as
fodder and sometimes processed into new products, e.g. straw rnats. Consequently, only such
agricultural "waste" that is not intended for some other use or for composting should be considered.
Most green plants are well-suited for anaerobic fermentation. Their gas yields are high, usually
above that of manure (cf. table 3.5). Wood and woody parts of plants resist anaerobic fermentation
and should therefore not be used in biogas plants. Due to the poor flow properties of plant material
and its tendency to form floating scum, it can only be used alone in a batch-type plant. In practice,
however, batch plants are unpopular because of the need for intermittent charging and emptying.
In continuous-type family-size biogas plants, crop residue therefore should only be used as an
addition to animal excrements. Any fibrous material like straw has to be chopped up to 2 - 6cm -
and even that does not fully preclude scum formation.
Table 3.4: Digestion characteristics of animal-husbandry residues (Source: OEKOTOP)
Substrate
Cattle manure
ditto, plus 10%
straw
Pig manure
ditto, plus 10%
straw
Chicken manure
Sheep/gcat
manure manure
Scum formation/
sedimentation
none
heavy
none
slight
slight to heavy
heavy to slight
heavy slight
slight to heavy
heavy
medium none
to
heavy
Digestion
very stable
very stable
Danger of "tilting",
i.e. acidification, at
the beginning; slow
run-up with cattle
manure necessary
ditto
Slow run-upwith
cattle
manure
advisable; danger
of "tilting"
stable
Recommend
ed retention
time (days)
60- 80
60-100
40 - 60
60 - 80
80
80-100
Gas yield
compared to
cattle manure
100%
120%
200%
...
200%
80%
18